1. Field of the Invention
The present invention relates generally to a method of growing a Group III-V compound semiconductor crystal layer such as GaAs from a solution, and more particularly it pertains to a method of doing so while controlling the conductivity type of the grown crystal layer as desired. Still more particularly, the present invention concerns a method of solution growth of a compound semiconductor crystal layer as mentioned above which uses a melt, serving as the solution, in which a Group IV element is doped as an impurity atom source for determining the conductivity type of the grown layer and by relying on the temperature difference technique while controlling the vapor pressure of the Group V element which constitutes said compound.
2. Description of the Prior Art
When a semiconductor device is manufactured, on a substrate, by the epitaxial growth of a III-V compound semiconductor crystal, which is typically GaAs, the manufacturing procedure relies primarily upon a solution growth method. The solution growth method, which has been generally employed conventionally for the growth of Group III-V compound semiconductors, is the so-called Nelson method which has been developed by RCA Corporation of the U.S.A. The Nelson method includes the steps of dipping a substrate crystal in a melt, which has been prepared using an appropriate metal as a solvent, and dissolving a Group III-V compound semiconductor crystal together with atoms of a required impurity type in the solvent. The resulting solution is slowly cooled thereby causing recrystallization of the supersaturating Group III-V compound semiconductor on the substrate. In this procedure crystal growth is conducted through a cooling process. Accordingly, the growth temperature varies in the thickness direction of the layer which is grown. Thus, this known method has the drawbacks that the amount of the impurity serving as the dopant would change with a decrease in the temperature, and that, in case of a mixed crystal, the composition of the crystal will change as well.
A typical example of this procedure is represented by the relationship between the growth temperature and the conductivity type of GaAs doped with Si, an amphoteric impurity which may produce an n type and a p type conductivity. When an Si-doped GaAs is grown relying on the conventional slow-cooling Nelson method mentioned above, the crystal grown at the high temperature zone will exhibit an n type conductivity, and the conductivity becomes lower as the growing temperature is lowered. When, via a high resistivity region, the temperature is lowered to a further extent, the conductivity type converts so that the crystal will now exhibit a p type conductivity. A method of forming a p-n junction in a single growth process by positively utilizing this phenomenon of conversion has been proposed. However, the presence of this phenomenon also serves to demonstrate that the grown layer obtained by this slow-cooling method is not uniform in its body.
The reason why the conversion of the conductivity type takes place has been attributed to the fact that, at a high temperature, atoms of Si easily enter substitutionally into the lattice sites of Ga atoms providing an n type conductivity, and that, at a low temperature, Si atoms easily enter substitutionally into the lattice sites of As atoms to thereby exhibit a p type conductivity.
Accordingly, this conventional pn junction forming method utilizing the different patterns of substitution of an amphoteric impurity such as Si into the lattice sites of a Group III-V compound semiconductor crystal, especially owing to the fact that the drawbacks of the so-called Nelson method of making a non-uniform crystal are positively made use of, will naturally develop poor crystal perfection, and the fluctuation of, for example, the cooling rate intensively affects the crystallographic quality of the layer which is grown. Thus, even when atoms of an amphoteric impurity are introduced as a dopant into a melt, there actually has not been carried out such an ideal lattice substitution that the impurity atoms enter successfully into the desired lattice sites.